1. Field of the Invention
This invention relates generally to improvements in the acceleration sensitivity of quartz crystal oscillators and more particularly to the elimination of accelerationinduced frequency shifts in such oscillators.
2. Description of the Prior Art
Quartz crystals, also called "crystals", "resonators" and "crystal resonators", are commonly used to control the frequency of electrical oscillators and in other circuits where an electrical resonant frequency is required. A major problem with such crystals is that their natural resonant frequency changes when acceleration forces are applied. The deleterious effects of these frequency shifts are well known to those skilled in the art.
There are two classes of methods for reducing the acceleration sensitivity of a crystal, namely active and passive. In active methods, an acceleration sensor and a feedback network is used to alter the oscillator frequency and thereby compensate for the acceleration induced frequency shifts. In passive methods, no attempt is made to sense the vibration or to dynamically change the output frequency.
One passive method of minimizing acceleration induced frequency shifts is disclosed in U.S. Pat. No. 4,365,182, entitled, "Method Of Fabricating Acceleration Resistant Crystal Resonators And Acceleration Resistant Crystal Resonators So Formed", issued to Arthur Ballato, et al. on Dec. 21, 1982. In that patent, the disclosed feature is that acceleration resistant resonators can be produced from a single quartz plate that is optically twinned into a left handed quartz portion and a right handed portion with the effective thickness of the two portions being equal. After depositing a pair of electrodes on each portion of the plate, the thickness of the electrode pairs is adjusted so that the resonant frequency of one or left hand (LH) portion is substantially the same as the resonant frequency of the other or right hand (RH) portion.
Another approach to the problem is disclosed in U.S. Pat. No. 4,344,010, entitled, "Acceleration Resistant Combination Of Opposite Handed Piezoelectric Crystals", which issued to John R. Vig, et al. on Aug. 10, 1982. That patent shows that acceleration sensitivity can be minimized by the use of two separate resonators, one left handed and one right handed, that are aligned such that all three crystallographic axes are anti-parallel.
In a more recent invention disclosed in U.S. Pat. No. 4,410,822, entitled, "Acceleration Resistant Crystal Resonator", issued to Raymond L. Filler on Oct. 18, 1983, a crystal resonator configuration is disclosed wherein the magnitude and direction of the acceleration sensitivity vectors of the two crystals are determined and a composite resonator is constructed by aligning two vectors in an anti-parallel relationship. When the acceleration sensitivity vectors of the two crystals are of equal magnitude, a more complete cancellation of vibrational effects is achieved for all acceleration directions.
Another approach to the acceleration sensitivity problem is taught in U.S. Pat. No. 4,451,755, entitled, "Acceleration Sensitivity Reduction Method", which issued to John R. Vig, et al. on May 29, 1984. In that patent, the acceleration sensitivity of an AT-cut quartz crystal resonator is reduced by replacing the quartz crystal plate of the resonator with a quartz crystal plate having a flatter plate contour thereupon the acceleration sensitivity coefficient decreases approximately linearly with the diopter value.
An active method of reducing the acceleration sensitivity of a crystal is disclosed in U.S. Pat. No. 4,453,141, entitled, "Suppression Vibration Effects On Piezoelectric Crystal Resonators", which issued to Vincent J. Rosati on June 5, 1984. What is taught there is the suppression or cancellation of the effects of vibration on quartz crystal controlled oscillators by generating an electrical signal which is a replica of the vibration acting on the crystal resonator, which signal is thereafter properly phased and applied directly to electrodes, which thereby operates to substantially eliminate unwanted vibration induced sidebands in the signal output of the oscillator.
It is an object of the present invention, therefore, to provide an improvement in the suppression of acceleration induced frequency shifts on crystal controlled oscillators.
It is another object of the invention to provide a substantially complete cancellation of vibrational effects on the crystal resonators of a crystal controlled oscillator.
And yet a further object of the invention is to provide a substantially complete cancellation of vibrational effects on the crystals of a crystal controlled oscillator as well as providing a substantially exact value of the desired oscillator frequency.